Light emitting diodes are widely used in consumer and commercial applications. As is well known to those having skill in the art, a light emitting diode generally includes a diode region on a microelectronic substrate. The microelectronic substrate may comprise, for example, gallium arsenide, gallium phosphide, alloys thereof, silicon carbide and/or sapphire. Continued developments in LEDs have resulted in highly efficient and mechanically robust light sources that can cover the visible spectrum and beyond. These attributes, coupled with the potentially long service life of solid state devices, may enable a variety of new display applications, and may place LEDs in a position to compete with the well entrenched incandescent lamp.
Group III nitride based LEDs, for example, may be fabricated on growth substrates (such as a silicon carbide substrates) to provide horizontal devices (with both electrical contacts on a same side of the LED) or vertical devices (with electrical contacts on opposite sides of the LED). Moreover, the growth substrate may be maintained on the LED after fabrication or removed (e.g., by etching, grinding, polishing, etc.). The growth substrate may be removed, for example, to reduce a thickness of the resulting LED and/or to reduce a forward voltage through a vertical LED. A horizontal device (with or without the growth substrate), for example, may be flip chip bonded (e.g., using solder) to a carrier substrate or printed circuit board, or wire bonded. A vertical device (without or without the growth substrate) may have a first terminal solder bonded to a carrier substrate or printed circuit board and a second terminal wire bonded to the carrier substrate or printed circuit board.
One difficulty in fabricating Group III nitride based LEDs on silicon carbide substrates has been the fabrication of high quality and low resistance epitaxial layers for LEDs. A gallium nitride layer (or other Group III nitride layer), for example, may be doped with an n-type dopant such as silicon to increase majority carrier concentration thereof and thereby reduce a forward voltage through a resulting LED. Doping with silicon, however, may increase lattice mismatch between the silicon doped gallium nitride layer and the silicon carbide growth substrate thereby increasing cracks in the gallium nitride layer (or other Group III nitride layer) and/or in epitaxial layers formed thereon. Reduced crystal quality due to increased silicon doping may increase forward voltage drop of the resulting LED and/or otherwise reduce performance. In other words, silicon doping provided to reduce resistance may reduce crystal quality (due to increased cracking) thereby reducing performance.
Accordingly, there continues to exist a need in the art to provide improved epitaxial Group III nitride layers for semiconductor devices such as LEDs, for example, by reducing forward voltage while maintaining and/or improving crystal quality.